1. Field of the Invention
The present invention relates to "private" remote control or communication systems of the type which may be employed to accomplish any of a plurality of functions at a controlled or satellite station in response to command signals radiated from a remotely located transmitter. More specifically, this invention is directed to the sequential transmission of a plurality of signals of different carrier frequency with modulation which varies in accordance with an operator selected code and the reception and decoding of the thus transmitted signals for the purpose of enabling or otherwise controlling the operation of equipment located at a remote receiver station. Accordingly, the general objects of the present invention are to provide novel and improved apparatus and methods of such character.
2. Description of the Prior Art
Multichannel remote control systems are well known in the art. U.S. Pat. No. 3,831,175, assigned to the assignee of the present invention, discloses a multichannel remote control of exceptional utility.
All remote control systems employing communications techniques, and particularly those which employ radio transmission, have been characterized by a certain degree of lack of security. This lack of security may, considering a very basic utilization of such systems, appear as inadvertent and/or unauthorized opening of garage doors or turning on of lights. In terms of more serious consequences, the interfering with a remote control system can result in such dire consequences as a detonation of explosives at an improper time and the connecting or disconnecting of important machinery to a power source at an inappropriate time.
Previous attempts to safeguard the privacy of transmission and reception of control signals have included complicated circuitry which will prevent the remote receiver from acting in response to spurious signals; such spurious signals either being in the form of "noise" or resulting from a deliberate attempt to exercise unauthorized control over equipment. Such prior art efforts to protect the privacy of remote control systems, to the limited extent that they have achieved the desired result of security, have been comparatively expensive and, due to circuit complexity, shomewhat lacking in reliability.
An additional consideration in remote control apparatus, particularly where the frequency spectrum employed by the apparatus is shared with other users, is to minimize transmission time and thus minimize interference with the transmissions of such other users; the other users typically transmitting on a single carrier frequency. Previous efforts to conserve the frequency spectrum and to minimize interference have, as in the case of prior attempts to enhance privacy, resulted in complicated and thus expensive circuitry.
A further and serious disadvantage of prior art remote control systems resides in the fact that changing of the transmitted "code" is, when possible at all, a difficult and comparatively time consuming task. There are numerous applications where the transmitter operator may desire to transmit a different code depending upon the immediate circumstances. By way of example, a police officer may wish to individually remotely turn on lights in commercial buildings for a brief period during patrol, each store being responsive to a separate code, and the officer may also wish to be able to "dial" a still further code indicating a need for assistance. All such transmissions should, of course, be "private" and not subject to jamming.